Energy dissipating devices are used to connect adjacent structural elements or systems of structures like a bridge deck and its abutment or a building and its foundation with the aim of mitigating the adverse effects of dynamic actions like those deriving from earthquakes but also from windstorms, impacts, etc.
As it is known, the reason for adopting these devices stems from the fact that seismic actions produce sudden onset soil displacements with associated accelerations that, transmitted through the foundation, generate inertial forces due to the comparatively large mass of structures such as bridges, buildings and the like.
Alternatively we may say that during a seismic event large amounts of energy are generated and propagated through the soil. Said energy can be transmitted to a structure, and the same represents the main cause of possible damage secondary to a seismic event.
Furthermore, it is also known that by using suitably located isolating devices, referred to as “seismic isolators”, it is possible to limit the amounts of energy transmitted to the structure and thus also the loads thereby generated. In other words, seismic energy is reflected by the aforesaid isolation devices, except for that portion associated with a frequency equal to or near the resonance frequency.
However, the structure (considered as an oscillating system) will nonetheless tend to accumulate the portion of energy within said spectrum range, making it thus necessary to use a device that dissipates the energy in the form of heat.
There are several physical mechanisms that can be used to dissipate energy. The subject case exploits the dissipating capacity of materials, when they are stressed beyond their yield point.
As it is well known, by subjecting for example a metal element like a strut made of steel to an increasing strain, a first phase of direct force-deformation proportionality is observed (elastic phase), followed by a phase of little dependence of the former on the latter (plastic or post-elastic phase). Devices using this principle are referred to as hysteretic elements (HE). Thus, the term “hysteretic” is referred to elements or devices, whose reaction largely depends on the impressed displacement (or deformation) in contrast to “viscous” elements or devices, whose reaction largely depends on the rate (velocity) of the displacement (or deformation).
The idea of utilizing steel struts as hysteretic elements within a structure to absorb large portions of seismic energy began with the conceptual and experimental work of Skinner—New Zealand (1975). Today this kind of device is known as steel hysteretic damper (SHD). Actually, plastic deformation of steel is one of the most effective mechanisms available for the dissipation of energy, from both economic and technical point of view. Steel dissipaters for steel hysteretic dampers have been conceived and manufactured in a very large geometric configuration variety. Nonetheless, their most serious drawback is the limited capacity of accommodating large displacements, as necessary in zones prone to moderate or high seismicity.
This is the main reason why hydraulic viscous dampers (HVDs) gained a progressively increasing popularity in bridge structures as elements capable of mitigating local vibrations, limiting structural drift as well as source of supplemental damping, virtually without any limitation displacement-wise.
In recent years, however, some concerns were raised in the United States by the conditions of devices in service on Caltrans bridges, suspected of leaking hydraulic fluid, after a time in service of approximately 10 years. Even though this event should not be interpreted as a proof of a systemic deficiency of this type of devices (in Europe such hitches are rarely been observed), nonetheless some tests and inspections, recently completed at University of California at San Diego, indicated an unexpected level of degradation in dampers not subjected to any large seismic event.
In response to this concern and as an alternative solution to the use of HVDs, a few research projects have been carried out at the University of Utah and the Department of Structural Engineering of University of California at San Diego to investigate the applicability and limitations of hysteretic elements (HEs) for long-span bridge applications.
Steel brace frames are widely used for building construction. For seismic applications, however, diagonal braces are expected to yield in tension, but buckle in compression. To avoid this undesirable phenomenon and thus provide a more reliable source of energy dissipation, the concept of buckling restrained braces (BRB) as hysteretic elements was first developed in Japan in the early 1970s.
Extensive research was since conducted in Japan and a variety of BRBs have been developed by Nippon Steel Corp. But it was not until after the Kobe Earthquake in 1995 that Japanese designers started to incorporate BRBs in the so-called “damage-tolerant” seismic design of multistory buildings. Actually, BRBs have also been used in two long-span bridges in Japan after the above mentioned disastrous event, one for a case of seismic retrofit and one for a new construction.
The main idea behind BRB is to avoid global buckling such that a full and stable hysteresis loop can be developed to dissipate energy. A variety of BRBs have been developed, but the concept is very simple. FIG. 11 illustrates this concept.
A long and relatively slender bar or strut functions as hysteretic element that acts as a yielding steel core, usually made of low strength steel with high ductility. It is encased in a steel tube with mortar infill functioning as a buckling preventer for the hysteretic element. However, as opposed to what occurs in reinforced concrete members, some unbonding materials or even an air gap is provided to isolate the yielding steel core and the surrounding buckling restraining mechanism to discourage composite action.
The foremost BRBs' drawback resides in their excessive length, which derives from the relatively limited permissible yield deformation under repeated cycling, as requested in seismic applications. This shortcoming severely limits the applicability of this type of devices to those cases where large spaces are available for their installation.
Therefore, it is an object of the invention to provide a simple and cheap energy dissipating device which has broad application possibilities even where space is limited.
The solution of this object is achieved by an energy dissipating device according to claim 1. Preferred further configurations of the energy dissipating device are described in the dependent claims.
It is characteristic for an energy dissipating device according to the invention, that the energy dissipating device comprises at least three hysteretic elements, being interconnected in series in such a way, that at least one hysteretic element will be subjected to compression while at least another hysteretic element is subjected to tension under an external load (e.g. seismic, wind and/or impact load) being applied to the energy dissipating device, as it will become clear in the following.
This has the beneficial effect that the axial overall dimension of the device is markedly reduced. Or, conversely, larger relative displacements can be accommodated in a device having equal length compared with those known from the past. By reducing the overall length of the device, it can be much more easily installed in existing or newly designed structures.
Moreover, the tensioned hysteretic element(s) may stabilize the compressed hysteretic element(s) against buckling, so that a buckling preventer may not be needed, at least under certain circumstances.
Furthermore, the energy dissipating device is preferably designed in such a way that it can undergo several hysteretic cycles before it needs to be replaced.
In another configuration of the energy dissipating device the hysteretic elements are arranged in parallel to each other. A parallel arrangement of the hysteretic elements has the advantage that said device has a compact overall length compared to existing Buckling Restrained Braces. Even though a parallel arrangement is preferred, here also such hysteretic elements are to be considered, which are arranged to form an angle between each other.
If a more compact device is desired, the hysteretic elements are preferably arranged alongside to each other. Since the at least three hysteretic elements are interconnected in series, the overall length of the energy dissipating device is about one third, compared to the overall length of an existing Buckling Restrained Brace with the same capacity to accommodate displacements. Another advantage is that a reduction of the overall length to one third leads to a buckling load that is at least increased by a factor of nine (9) according to Euler's theory. This is a further reason why a buckling preventer may not be needed in this type of device. Consequently, it is also possible to increase the capacity to accommodate displacements by a factor of about three (3) compared to existing Buckling Restrained Braces with an equal overall length.
Alternatively, at least one hysteretic element is arranged concentrically with another hysteretic element. The concentric arrangement of a hysteretic element does not necessarily require that the center axes of the hysteretic elements coincide in a line. Although this is preferred, also such energy dissipating devices shall be included in which the center axes of the hysteretic elements do not coincide. By this arrangement it is also possible to reduce the length of the energy dissipating device. This has the aforementioned advantages of a reduced overall length compared with a conventional Buckling Restrained Brace having the same capacity to accommodate displacements.
Preferably, the energy dissipating device may have at least one hysteretic element that is made of a metal like mild steel or an alloy. Since the amount of energy which can be dissipated by the device as well as the ability to accommodate displacements depends on the hysteretic element's material, here especially materials with a high ductility, which preferably also show similar deformation characteristics under compression and tension, are preferred. By a suitable choice of the hysteretic element's material, as well as by the choice of the hysteretic element's dimensions, design parameters such as the yield point, the capability of accommodating displacements and the capability of dissipating energy by the hysteretic elements can be influenced.
It is practical that concentrically arranged hysteretic elements are formed of tubes. The tubes may have a cross section area which has a circular, square, rectangular or any other closed or opened shape. However, tubes with a closed and symmetrical cross sectional shape are preferred.
Symmetrical tubes do increase the resistance against buckling since lateral or radial deformations under axially applied loads are minimized. One significant benefit accrued from the use of tubes is the elimination of the undesirable effects of the bending moment M=F*e that develops during the application of an axial force “F” when using for example flat steel bars as hysteretic elements with an eccentricity “e”.
Obviously, to be concentrically arranged the inner tubes have a smaller diameter than the outer tube. The cross sectional area should be the same for all concentric tube-like hysteretic elements and is determined by the design reaction force of the device and the yield strength of the material used. This means, apart from some minor corrections due to the shape factor, the thickness of the tube's wall is inversely proportional to its diameter. As a result the concentrically arranged tubes have a similar deformation behavior.
Moreover the median part of one or more tubes may be cut longitudinally, in other words, along a generatrix. This helps to remedy the drawbacks of radial deformations of the tubes induced by axially applied loads. It is known that axially applied loads in tube-like elements produce tangential stresses in the same, which cause reduction or increase in their diameter in tension or compression respectively. Consequently, two adjacent tubes may get in touch and produce uncontrollable frictional forces, which negatively influence the reaction of the device. This effect is particularly important for configurations where the tube is arranged in the middle of adjacent concentrically arranged tubes. The local instability of the lips resulting from the cutting is minimized by the containment effect of the surrounding outer and inner tubes.
Further, the energy dissipating device may comprise a buckling preventer. A buckling preventer may be any contrivance that helps to avoid buckling of a hysteretic element. Although it is preferred to avoid the use of a buckling preventer, it might be necessary to provide an energy dissipating device with a buckling preventer to reduce the risk of buckling particularly in specific situations.
The innermost of at least one hysteretic element may be filled with a stabilizing material as buckling preventer.
As stabilizing material every high compression resistant material is suitable, for instance a mortar infill. The stabilizing material is an effective and economic method to avoid lateral deformations of said hysteretic element and in consequence to increase the resistance against buckling of the whole device.
In a further configuration of the invention's energy dissipating device the buckling preventer comprises a containment tube surrounding at least one hysteretic element wherein the space between the hysteretic element and the containment tube may be filled with a stabilizing material at least partially. By the use of a containment tube a lateral deformation of the at least one hysteretic element under an axially applied load is avoided. By filling a stabilizing material in the space between the hysteretic element and the containment tube the possibility of a lateral deformation of the at least one hysteretic element under an axially applied load is even more reduced since there is no space left for such a deformation.
Further the buckling preventer may also comprise a containment tube which surrounds several hysteretic elements at once. Then the space between the hysteretic elements and/or the containment tube and/or the space between the hysteretic elements themselves is preferably filled with a stabilizing material.
The containment tube may be manufactured from a single piece of tube. Alternatively, the buckling preventer may also be composed of an outer containment tube and an inner containment tube. In this case the space between the outer and inner containment tube is preferably filled with a stabilizing material as additional buckling restraining mechanism. The use of an inner and outer containment tube, especially if the space in-between is filled with a stabilizing material, is a simple but effective way of increasing the buckling resistance of the energy dissipating device.
Especially in the case where the hysteretic elements are symmetrically located with respect to the longitudinal axis of the device, the buckling preventer may comprise at least one transverse restraint interconnecting at least two hysteretic elements. Such restraints prevent the lateral deformation of the at least two hysteretic elements, thus the buckling resistance of the device is increased. Moreover, by using more transverse restraints connecting outer ends as well as the intermediate parts of both hysteretic elements, it is possible to further increase the buckling resistance of the whole device. In this way an external containment tube used as buckling preventer may be avoided. This leads to a simpler construction of the energy dissipating device.
In a further configuration, the at least one hysteretic element may comprise on at least one of its surfaces a lubricant and/or an un-bonding material and/or a sliding pad in order to reduce frictional forces developed during a relative movement between the at least one hysteretic element and the stabilizing material and/or the containment tube and/or another hysteretic element under an external load. This is based on findings that it is advantageous if the hysteretic elements themselves have no forced contact with the stabilizing material so that they are able to move in direction of the axially applied load without excessive frictional forces.
Especially in the case, when the hysteretic elements are tube-like elements, the energy dissipating device may comprise a rigid joint which connects at least two hysteretic elements in series, being formed by at least a steel crown and/or steel plate interconnecting the two hysteretic elements. The advantage of the rigid joint is that with its stiffness it is able to resist radially applied forces. Consequently the steel crown and/or steel plate interconnecting the two hysteretic elements reduce radial deformations (changes in diameter) of the hysteretic elements. Thus a contact as well as frictional forces between the tube-like elements are avoided.
Further, the energy dissipating device may comprise a shock-transmitter that accommodates slow velocity movements and transmits almost unaltered sudden onset movements. A shock-transmitter allows slow movements without appreciable resistance, but prevents those of sudden onset without appreciable deformations. Thus, this arrangement enables to accommodate the displacements between the interconnected structural elements due to thermal variations and to transmit to the hysteretic elements the displacements due to an earthquake (but also braking forces, wind etc.) so that a significant portion of the energy associated to them can be dissipated.
Within FIG. 1a to FIG. 10 equal reference signs are used for equal components.